Potassium-argon dating
Encyclopedia
Potassium–argon dating or K–Ar dating is a radiometric dating
method used in geochronology
and archeology. It is based on measurement of the product of the radioactive decay of an isotope
of potassium
(K) into argon
(Ar). Potassium is a common element found in many materials, such as mica
s, clay minerals
, tephra
, and evaporites. In these materials, the decay product 40Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallises
). Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar accumulated to the amount of 40K remaining. The long half-life
of 40K allows the method to be used to calculate the absolute age
of samples older than a few thousand years.
The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature
of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.
of to 40Ca and 40Ar. Conversion to stable 40Ca occurs via electron emission (beta decay
) in 89.1% of decay events. Conversion to stable 40Ar occurs via positron emission (inverse beta decay, electron capture
) in the remaining 10.9% of decay events.
Argon, being a noble gas
, is a minor component of most rock samples of geochronological
interest: it does not bind with other atoms in a crystal lattice. When 40K decays to 40Ar, the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. But it can escape into the surrounding region when the right conditions are met, such as change in pressure and/or temperature. 40Ar atoms are able to diffuse through and escape from molten magma because most crystals have melted and the atoms are no longer trapped. Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again. After the recrystallization of magma, more 40K will decay and 40Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified.
Calcium is common in the crust, with 40Ca being the most abundant isotope. Despite 40Ca being the favored daughter nuclide, its usefulness in dating is limited since a great many decay events are required for a small change in relative abundance, and also the amount of calcium originally present may not be known.
The scale factor 0.109 corrects for the unmeasured fraction of 40K which decayed into 40Ca; the sum of the measured 40K and the scaled amount of 40Ar gives the amount of 40K which was present at the beginning of the elapsed time period. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required.
of the gases released when a rock sample is melted in flame photometry or atomic absorption spectroscopy
.
The amount of 40K is rarely measured directly. Rather, the more common 39K is measured and that quantity is then multiplied by the accepted ratio of 40K/39K (i.e., 0.0117%/93.2581%, see above).
The amount of 36Ar may also be required to be measured.
Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample. Ar–Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem.
applications, it plays an important role in archaeology
. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge
by dating lava
flows above and below the deposits. It has also been indispensable in other early east Africa
n sites with a history of volcanic activity such as Hadar, Ethiopia
. The K–Ar method continues to have utility in dating clay mineral diagenesis
. Clay minerals are less than 2 micrometres thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
method used in geochronology
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
and archeology. It is based on measurement of the product of the radioactive decay of an isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(K) into argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
(Ar). Potassium is a common element found in many materials, such as mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
s, clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...
, tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
, and evaporites. In these materials, the decay product 40Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallises
Recrystallization (geology)
In geology, solid-state recrystallization is a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same...
). Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar accumulated to the amount of 40K remaining. The long half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 40K allows the method to be used to calculate the absolute age
Absolute dating
Absolute dating is the process of determining an approximate computed age in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision...
of samples older than a few thousand years.
The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.
Decay series
Potassium naturally occurs in 3 isotopes – 39K (93.2581%), 40K (0.0117%), 41K (6.7302%). The radioactive isotope 40K decays with a half-lifeHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of to 40Ca and 40Ar. Conversion to stable 40Ca occurs via electron emission (beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
) in 89.1% of decay events. Conversion to stable 40Ar occurs via positron emission (inverse beta decay, electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
) in the remaining 10.9% of decay events.
Argon, being a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
, is a minor component of most rock samples of geochronological
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
interest: it does not bind with other atoms in a crystal lattice. When 40K decays to 40Ar, the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. But it can escape into the surrounding region when the right conditions are met, such as change in pressure and/or temperature. 40Ar atoms are able to diffuse through and escape from molten magma because most crystals have melted and the atoms are no longer trapped. Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again. After the recrystallization of magma, more 40K will decay and 40Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified.
Calcium is common in the crust, with 40Ca being the most abundant isotope. Despite 40Ca being the favored daughter nuclide, its usefulness in dating is limited since a great many decay events are required for a small change in relative abundance, and also the amount of calcium originally present may not be known.
Formula
The ratio of the amount of 40Ar to that of 40K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the following equation:- t is time elapsed
- t1/2 is the half-lifeHalf-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 40K - Kf is the amount of 40K remaining in the sample
- Arf is the amount of 40Ar found in the sample.
The scale factor 0.109 corrects for the unmeasured fraction of 40K which decayed into 40Ca; the sum of the measured 40K and the scaled amount of 40Ar gives the amount of 40K which was present at the beginning of the elapsed time period. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required.
Obtaining the data
To obtain the content ratio of isotopes 40Ar to 39K in a rock or mineral, the amount of Ar is measured by mass spectrometryMass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
of the gases released when a rock sample is melted in flame photometry or atomic absorption spectroscopy
Atomic absorption spectroscopy
Atomic absorption spectroscopy is a spectroanalytical procedure for the qualitative and quantitative determination of chemical elements employing the absorption of optical radiation by free atoms in the gaseous state. In analytical chemistry the technique is used for determining the concentration...
.
The amount of 40K is rarely measured directly. Rather, the more common 39K is measured and that quantity is then multiplied by the accepted ratio of 40K/39K (i.e., 0.0117%/93.2581%, see above).
The amount of 36Ar may also be required to be measured.
Assumptions
According to the following assumptions must be true for computed dates to be accepted as representing the true age of the rock- The parent nuclide, , decays at a rate independent of its physical state and is not affected by differences in pressure or temperature. This is a well founded major assumption, common to all dating methods based on radioactive decay. Although changes in the electron capture partial decay constant for possibly may occur at high pressures, theoretical calculations indicate that for pressures experienced within a body of the size of the Earth the effects are negligibly small.
- The / ratio in nature is constant so the is rarely measured directly, but is assumed to be 0.0117% of the total potassium. Unless some other process is active at the time of cooling, this is a very good assumption for terrestrial samples.
- The radiogenic argon measured in a sample was produced by in situ decay of in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. Well-known examples of incorporation of extraneous include chilled glassy deep-sea basalts that have not completely outgassed preexisting *, and the physical contamination of a magma by inclusion of older xenolitic material. The Ar–Ar dating method was developed to measure the presence of extraneous argon.
- Great care is needed to avoid contamination of samples by absorption of nonradiogenic from the atmosphere. The equation may be corrected by subtracting from the measured value the amount present in the air where is 295.5 times more plentiful than . decayed = measured − 295.5 × measured.
- The sample must have remained a closed system since the event being dated. Thus, there should have been no loss or gain of or *, other than by radioactive decay of . Departures from this assumption are quite common, particularly in areas of complex geological history, but such departures can provide useful information that is of value in elucidating thermal histories. A deficiency of in a sample of a known age can indicate a full or partial melt in the thermal history of the area. Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories.
Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample. Ar–Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem.
Applications
Due to the long half-life, the technique is most applicable for dating minerals and rocks more than 100,000 years old. For shorter timescales, it is likely that not enough Argon 40 will have had time to accumulate in order to be accurately measurable. K–Ar dating was instrumental in the development of the geomagnetic polarity time scale. Although it finds the most utility in geologicalGeology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
applications, it plays an important role in archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge
Olduvai Gorge
The Olduvai Gorge is a steep-sided ravine in the Great Rift Valley that stretches through eastern Africa. It is in the eastern Serengeti Plains in northern Tanzania and is about long. It is located 45 km from the Laetoli archaeological site...
by dating lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows above and below the deposits. It has also been indispensable in other early east Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
n sites with a history of volcanic activity such as Hadar, Ethiopia
Hadar, Ethiopia
Hadar is a village in Ethiopia, on the southern edge of the Afar Triangle with a latitude and longitude of approximately . The village is known for the nearby archaeological site....
. The K–Ar method continues to have utility in dating clay mineral diagenesis
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
. Clay minerals are less than 2 micrometres thick and cannot easily be irradiated for Ar–Ar analysis because Ar recoils from the crystal lattice.